Question

void *p = malloc(1000);
*((int*)p) = 666;
*((int*)p+sizeof(int)) = 777;
int i;
for (i = 0; i<10; ++i)
    printf("%d ", *((int*)p+sizeof(int)*i));

Is the manual offset being resolved at compile time or does it add overhead of performing arithmetic operations during runtime?

Answer 1

Even if you have a constant instead of sizeof(int), compiler cannot know in advance the address in p, so it will have to do the addition. If you have something like i = sizeof(int)+4 then it should do the optimization compile time and directly set i to 8.

Also, I think when you do:

*((int*)p+sizeof(int)) = 777;

what you mean is:

*((int*)p + 1) = 777; /* or ((int*)p)[1] = 777; */

similarly printf("%d ", *((int*)p+sizeof(int)*i)); should be:

printf("%d ", *((int*)p + i));